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Publication numberUS3698635 A
Publication typeGrant
Publication dateOct 17, 1972
Filing dateFeb 22, 1971
Priority dateFeb 22, 1971
Also published asCA960032A1, DE2207310A1, DE7205829U
Publication numberUS 3698635 A, US 3698635A, US-A-3698635, US3698635 A, US3698635A
InventorsSickles James E
Original AssigneeRansburg Electro Coating Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Spray charging device
US 3698635 A
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Description  (OCR text may contain errors)

United States Patet Sickles [451 Oct. 17, 1972 [54] SPRAY CHARGING DEVICE [72] Inventor: James E. Sickles, Indianapolis, Ind.

[73] Assignee: Ransburg Electro-Coating Crp., In-

clianapolis, Ind.

[22] Filed: Feb. 22, 1971 [21] Appl. No.: 117,494

[57] ABSTRACT An electrostatic spray charging device wherein means in a passageway or a barrel of such device directs a jet :LS. Cl ..239/3, 239/ of between a charged electrode in the passageway and a plurality termini at an orifice within the l 1 0 can 15 317/3 1 passageway in a manner as to direct particles atomized 8 from the plurality of termini generally away from the charged electrode and the side walls of the [56] References Clted passageway. The charged electrode and the liquid ter- UNTTED STATES PATENTS mini are adapted to be connected to an electrical source thereby creating a potential difference between 3,606,972 9/1971 Ferrant ..239/1 5 h charged electrode and the q i i i The 3,3 1 7,] 1 Fraser 5 potential difference therebetween p i a 5 8; 2 3 Badger "239/15 tensity electric field at the liquid termini whereby pargg gg 1 i 1 2 gowen et ticles atomized from the individual liquid termini carry 85 1111942 fi' 00 7 a high charge-to-mass ratio. Preferably, the jet of air 288l092 4/1959 g X passing between the charged electrode and the liquid 3049O92 8/1962 i "5 X termini assists in atomizing liquid particles from the 3l79849 4/1965 e a "317/3 UX liquid termini. The charged liquid particles are at- C warmer tracted to and deposited upon the surface of an object FOREIGN PATENTS OR APPLICATIONS or'article at a particle attracting potential.

1,358,199 3/1964 France ..239/15 15 Claims, 4 Drawing Figures 26 2| u(3|/,?32 ////I//////// i 23 PATENIED I 17 I972 3.698.635

sum 2- or 2 llll //jI//////A//////// INVENTOR JAMES E. SICKLES SPRAY CHARGING DEVICE This invention generally relates to a low voltage electrostatic spray device that provides a spray of electrostatically charged particles, and more particularly, to a low voltage electrostatic spray device wherein liquid particles are formed and electrostatically charged in a passageway of the spray device and are directed, by a jet of air, away from the side walls of the passageway and an electrode cooperatively associated with the side walls of the passageway.

The electrostatic spray device of the present invention is intended primarily for use in the electrostatic application of liquid particles to the surfaces of objects or articles of a particle attracting potential to thereby provide such surfaces with a layer of the liquid particles, and, preferably with a substantially continuous film of coating of the liquid particles.

Several systems are commercially available which employ electrostatic principles to aid in the deposition of liquid particles upon an article to be coated. These systems differ from each other in several respects including the means and methods used to atomize and electrostatically charge the atomized liquid particles.

One electrostatic spray system known as the Ransburg No. 1 Process" uses a conventional metal air atomizing spray gun to form uncharged liquid particles. A separate grid of highly charged wires is employed to impart to the atomized liquid particles an electrostatic charge. In this system an electric field of high intensity exists between the grid of charged wires and the article surface to be coated. The electric field creates an abundance of atmospheric ions of the same electrical polarity as the grid. The atmospheric ions are directed toward the article surface at ground potential. The liquid particles are formed by the spray gun and projected into the ion cloud where they are electrostatically charged by ion bombardment. The charged liquid particles are attracted to and deposited upon the surface of the article in response to the interaction of the charge carried by the liquid particles, the electric field and the grounded surface of the article.

There are a number of other systems which are similar to the Ransburg No. l Process in that electrostatic charging of the liquid particle is accomplished by ion bombardment. The systems differ from one another in respect to the location of the ionizing electrode relative to the liquid particle forming means and to the surface of the article to be coated.

In another system, the Ransburg No. 2 System, a high voltage deposition and charging electric field is established between the article surface to be coated and an exposed body of liquid material formed on an atomizer having an extremely thin edge portion. The exposed body of liquid material is formed so as to be coextensive with the extended thin edge portion of the atomizer. The electric field has a high gradient at the extended edge of the liquid. The electric field forms the extended edge of the liquid into a number of fluid termini or cusps which extend outwardly. Each of these termini are highly charged and, therefore, a strong electrical repulsion exists between the main liquid body and the liquid at the termini. Conversely, a strong electrical attraction exists between the liquid at the termini and the surface of the article at a particle attracting potential. Therefore, a small portion of the liquid at each of the termini is aided in escaping the forces of surface tension of the liquid. The small portion of liquid is pulled into the electric field as a highly charged particle to be deposited on the surface of the article at a particle attracting potential.

In still another system for electrostatically charging liquid particles, a spray gun is constructed of substantially dielectric material and uses a wire-like electrode projecting from a liquid orifice. The electrode charges air atomized liquid particles by bombardment with atmospheric ions created at the tip of electrode. The atmospheric ions are concentrated in the immediate vicinity of the tip of the electrode. Ion bombardment of the atomized liquid particles more completely charges the liquid particles. The wire-like electrode also provides an electrostatic field extending from its tip to the surface of the object or article to be coated. The electrostatic field assists in promoting the deposition of the charged liquid particles upon the surface of the object or article. To provide the necessary atmospheric ions, the wire-like electrode is connected to a direct voltage source capable of supplying a no load output voltage of up to about 65,000 volts at a short circuit current at the electrode of up to about 225 microamperes. It is apparent that voltages of such magnitude create safety problems. Furthermore, the power supply necessary to provide such high voltages is, bulky and expensive. In the event the electrostatic spray device is to be manipulated by hand, the voltage cable necessary to supply such high voltages to the device tends to be stiff and heavy and thus inhibiting free manipulation of the spray device and causing operator fatigue.

Each of the above electrostatic devices and systems has attained a high degree of technological and commercial success. Each of the devices uses a voltage of high magnitude, each device differs from the other devices in the magnitude of the charge imparted to the liquid particles, each device differs from the other devices in the types of liquid materials each is capable of spraying efficiently, and each device differs from the other devices general size, weight and adaptability to a specific use. I

It is, therefore, a desideratum to have an electrostatic spraying device that will spray a wide variety of different types of liquid materials without regard to physical and electrical characteristics of such liquid materials; that will be light in weight and readily maneuverable; that will not subject the operator to harmful shock or discharge to grounded surfaces during operation; and that will form particles of such size as will exhibit a high charge relative to the mass of the particles so that electrostatic deposition will occur at relatively high efficiency. Generally, liquid particles carrying a high charge-to-mass ratio are more readily attracted to and deposited on a surface at particle attracting potential than are liquid particles having a low charge-to-mass ratio. In most instances, the composition of the liquid material and, therefore, its density, is determined by intended use of the liquid material. For a particle of given size and density maximizing the charge-to-mass ratio of the particle is a question of maximizing the charge carried by the particle. It is highly desirable, therefore, to provide a spray device possessing the capability of producing highly charged particles while having the other desired characteristics.

The present invention provides a relatively low voltage electrostatic spray device capable of imparting very high electrostatic charge to atomized liquid articles. The spray device possesses a nonionizing electrode located within and spaced at a distance from the end of a passageway formed within a barrel. Within the passageway and in close proximity to the electrode is located the termini of a liquid supply. Connected to the termini of the liquid supply and the electrode is a means for establishing a high intensity electric field therebetween. Preferably, the side walls of the passageway except for the electrode portion are fabricated from a dielectric material to thereby concentrate the electric field at the terminus of the liquid supply. The electrode and the side walls of the passageway are substantially continuously swept by a jet of air which carries the particles out of the passageway. Sweeping the electrode and side walls of the passageway with a jet of air significantly reduces the number of liquid particles which would otherwise accumulate on the electrode and side walls of the passageway. If liquid particles were allowed to accumulate to any significant degree on either the electrode or the side walls of the passageway, undesirably large liquid droplets may be formed from such an accumulation. The presence of undesirably large droplets in a spray of charged liquid particles tends to impair the finish usually desired on the surface of an object or article and will likewise lower the charge-to-mass ratio created on such particles. In addition, locating the electrode in close proximity to the termini of the liquid supply and at a potential different from the potential of the electrode and forming the side walls of the passageway from a dielectric material permits the use of a low voltage source to create a high intensity field at the termini formed at the surface of the stream of liquid.

The electrical gradient, as high as is practicable, maintained between the electrode and the termini of the stream of liquid in the passageway, creates an electrostatic field of high intensity at each termini. Liquid particles formed from the liquid termini possess a high charge and are projected from the spray device by the jet of air and are attracted to the surfaces of objects or articles maintained at a particle attracting potential. Preferably, the jet of air also aids in the atomization of particles from the liquid termini in the passageway, the liquid termini being within a few ten thousandths of an inch or less of the electrode. The spray device has no exposed or external metallic electrode thereby reducing the possibility of an operator contacting the metallic electrode or the electrode engaging the surfaces of articles or objects adapted to receive the charged liquid particles. The internal location of the electrode within the spray device increases the safety associated therewith.

The present invention also provides an electrostatic spray device which employs direct current input voltages to the electrode of about 10,000 volts or less and preferably about 7,000 volts or less and low value input currents of about 20 microamperes or less and preferably about to 5 microamperes or less. The use of a low value input voltage and current to the spray device permits a significant reduction in the size and weight of the power supply used to provide the field intensity necessary to appropriately charge the liquid particles. It is contemplated the physical size of the power supply will be such as to permit it to be located either within the handle of the electrostatic spray device or on the person of the operator, if desired. Locating the power supply within the spray device or on the person of the operator significantly increases the portability, ease of manipulation and applicability of such a device. It is to be understood, however, that if operating conditions and/or regulations so require, the power supply of the spray device may be located remotely from the site at which spraying is to be undertaken.

The appended drawings are intended to illustrate several spray devices embodying the concepts of the present invention constructed to function in the most advantageous modes presently devised for-the practical application of the principles involved in the hereinafter described invention.

In the drawings:

FIG. 1 is a diagrammatic illustration of an electrostatic spray device embodying the concepts of the present invention wherein the power supply is preferably incorporated within the spray device;

FIG. 2 is an enlarged partial cross sectional side view of the front end of a barrel intended to be used with the spray device illustrated in FIG. 1;

FIG. 3 is an enlarged side view showing the spray device in FIG. 1 including a spray-surrounding electrode; and

FIG. 4 is an enlarged partial cross sectional side view of the front end of a barrel illustrating another form of a spray device embodying the concepts of the present invention.

Referring now to FIG. 1 of the drawing, an electrostatic spray device or gun incorporating the concepts of the present invention is indicated by the reference numeral 10. The spray gun 10 includes tubular means or barrel ll, handle 12 extending from the barrel at an angle thereto, and trigger l7. Handle 12 may contain a suitable battery-operated, direct current power supply (not shown) capable of supplying up to about 10,000 volts to a metallic electrode located within the barrel 11 of spray gun l0.

Conduits l3 and 14 project from the lower extremity of the handle 12 of spray gun 10. Conduit 13 connects the gun to air source 15 capable of supplying air in suitable volumes to spray gun 10 for sweeping charged particles of liquid from the gun. Conduit 14 connects spray gun 10 to reservoir 16 containing a liquid material to be atomized, charged and deposited on the surface of an object or article. The liquid contained within reservoir 16 may be, for example, a functional liquid capable of being atomized such as paint, varnish, lacquer, emulsions or the like diluted, if necessary, with a suitably conductive solvent or admixture of solvents that are chemically compatible with the liquid to be sprayed. Solvents, such as ketones, alcohols, ethers and the like are preferred. However, it is to be understood that not all the solvents in the above chemical groups are chemically compatible with all the named liquid materials, but all groups of the listed solvents include specific solvents within each group that are compatible with, for example, paint.

Trigger 17, pivotally carried by barrel 11 of spray gun 10, regulates the supply of air and liquid to the gun, and the voltage supplied to the metallic electrode in any suitable, known manner. Since this regulation mechanism may be of any known suitable form, of which many are conventionally used, it has been omitted from the drawing in the interest of a clearer showing of the inventive portion of the spray gun 10. Preferably, the exposed external components of the spray gun such as handle 12 and trigger 17, fabricated from conductive materials such as metal, are electrically grounded or earthed.

Referring now to FIG. 2, an enlarged partial cross sectional side view of the front end of a barrel 1 1 incorporating the concepts of the present invention is shown. Barrel 1 1 includes a substantially tubular sheath or housing 18 having an integral, inwardly extending radial flange 19 at its forwardmost extremity. Flange 19 provides opening 20 through which a spray of electrostatically charged liquid particles (not shown) is projected. Housing 18 is fabricated from any suitable conductive material such as metal or the like or from any dielectric material such as acetal resin and the like. In the event the housing is fabricated from a conductive material, it is, preferably, earthed or grounded.

Outer tube and inner tube 26 are located within barrel housing 18 and are components of 11. Preferably, tube 25 and tube 26 are concentric and both are fabricated from any suitable dielectric material capable of withstanding the stresses associated with the highest voltages provided by the power supply without an accompanying breakdown or rupture of the material. A suitable insulative material for tubes 25 and 26 is acetal resin, epoxy, glass filled epoxy, glass filled nylon and the like.

Tube 26 includes at its forward extremity, an axially movable bullet-shaped cap 27. Preferably, cap 27 is fabricated from a dielectric material such as acetal resin. The rear surface 29 of cap 27 is substantially parallel to and spaced from extremity 32 of tube 26. Surface 29 and extremity 32 are substantially perpendicular to the exposed surface of metallic electrode 31. Extremity 32 of tube 26 and rear surface 29 of cap 27 cooperate so as to provide an annular discharge orifice 30 through which a stream of liquid issues. The stream of liquid is caused to flow to the region near the peripheral annular edge of rear surface 29 of cap 27 by the application of sufficient pressure to the liquid in reservoir 16. Air moving through annular passageway 21 formed by the structural cooperation relationship existing between tube 25 and tube 26 aids in forming the liquid into numerous liquid termini near the radial extent of rear surface 29 of cap 27 and carries the particles atomized from the termini forwardly through opening 23.

Preferably, the surface 29 of cap 27 is axially movable with respect to extremity 32 to thereby provide an orifice 30 with a variable annular opening to thereby assist in regulating the flow of liquid from the liquid reservoir 16 to the liquid termini. As illustrated in FIG. 2, the rear surface 29 of cap 27 has associated therewith a diameter of greater extent than the diameter of adjacent tube 26. Operation of the gun with such dimensional relationships causes the periphery of the rear surface 29 of the cap 27 to provide an annular edge 28 near which the liquid termini are formed.

It has been observed that the degree to which the rear surface 29 of cap 27 radially extends beyond the extremity 32 of tube 26 is also important to the efficient charging of the liquid particles. The optimum difference between such dimensions depends on, among other things, the contribution of the jet of air to the atomization process and the resistivity of the liquid to be atomized and charged. For example, liquid resistivities in the range of about 1.6 megohm-centimeters to about 13 megohm-centimeters experience more charging when rear surface 29 of cap 27 has a radial extent about 5 to 8 percent greater than the radial extent of extremity 32 of tube 26. Increasing the difference in radial extent of rear surface 29 over the radial extent of extremity 32 to about 10 percent or more does not appear to harrnfully affect the atomization characteristics of the liquid; however, the charging of the liquid particles is considerably reduced. It is to be understood that the differences in radial extent vary as the resistivity of the liquid varies from the liquid resistivities given above. For example, if the resistivity of the liquid is about 1.5 megohm-centimeters or less, optimum charging of the liquid particles occurs when the radial extent of rear surface 29 of cap 27 and the radial extent of extremity 32 of tube 26 are substantially equal.

Another important dimensional relationship is the distance between edge 28 of cap 27 and electrode 31. The optimum distance for good charging of the liquid particles is about 0.010 to 0.025 of an inch (0.025-0.063 cm) and, preferably, about 0.015 to 0.020 of an inch (0.038-0.05l cm). Increasing this distance beyond about 0.025 of an inch (0.063 cm) significantly reduces the charge carried by the liquid par ticles projected from barrel 1 1.

Annular electrode 31 having extended axial length is embedded in tube 25 as shown in FIG. 2. The leading edge of electrode 31 is adjacent but removed rearwardly from opening 23 of tube 25. Electrode 31 may be coupled through current limiting resistor 41 of suitable ohmic resistance to direct current power supply 22.

The ohmic value of the current limiting resistor varies depending on the magnitude of the voltage appe aring at the output terminal of the power supply. For example, a direct current voltage source providing an output of about 4,000 volts requires a current limiting resistor having an ohmic value of about 50 to 60 megohms to limit the short circuit current to about to microamperes whereas a source providing an output voltage of about 10,000 volts requires a current limiting resistor having an ohmic value of about to megohms to limit the short circuit current to about 70 to 80 microamperes. Power supply 22 may be located within spray gun 10 or on the person of the operator of the spray gun. Alternatively, the resistor 41 may be removed and electrode 31 may be directly connected to power supply 22. The voltage supplied to electrode 31 by power supply 22 creates a high intensity electric field extending between electrode 31 and the terminus of the liquid present at annular orifice 30 adjacent to annular edge 28 of bullet-like cap 27.

The configuration of cap 27 is important. The con figuration of cap 27 should be such as not to inhibit or impede the projection of liquid particles from the spray gun 10 to the surface of the article at a particle attracting potential. The configuration of the bullet-like cap 27 allows the air-liquid particle mixture to expand into an area of increasing volume thereby assisting in directing the liquid particles away from electrode 31 and the side walls of tube 25. Sufficient turbulence and reduction in pressure appears near the side walls of tube 25 to discourage the vast majority of particles from accumulating thereon. The reduction in air pressure near the side walls of tube 25 draws most of the particles, if any, deposited thereon from the side walls prior to any harmful accumulation thereof.

The distance between the electrode 31 and edge 28 of cap 27 is a fraction of an inch. Preferably, the distance between electrode 31 and edge 28 is in the order of about 0.020 of an inch (0.051 cm) or less. Preferably, the distance between electrode 31 and the exterior surface of tube 26 is on the order of 0.030 of an inch (0.076 cm). An electrostatic field of sufficient strength to provide the liquid particles with a high charge-to-mass ratio extends to the termini of the liquid provided by stream near edge 28. The electrostatic field is provided by, preferably, a direct current voltage of about 10,000 volts or less, and preferably, about 3,000 to 7,000 volts being applied to electrode 31 and the termini of the stream of liquid being grounded through head 24 connected to the column of liquid in the hollow interior 34 of tube 26. The average linear voltage gradient associated with a distance 0.020 of an inch (0.05] cm) between electrode 31 at about 4,000 volts and the termini of the liquid at about ground potential is about 200,000 volts per inch (80,000 volts per cm). It is seen that liquid particles formed from the liquid termini near edge 28 are formed in a region of very high electric field strength. Since the material forming the orifice 30 is dielectric, the electric field lines tend to be concentrated at the conductive liquid termini which produces very efficient charging of the liquid particles. The materials forming the orifice 30 should be non-conductive, otherwise the charging efficiency of the device is substantially reduced.

The highly charged liquid particles projected through opening from the opening 23 and are attracted to surfaces (not shown) of articles or objects maintained at a particle attracting potential, preferably, ground or earth potential.

The foregoing and the following dimensions and parameters are given to illustrate the operation of the barrel shown in FIG. 2 with the spray gun shown in FIG. 1. Such dimensions and parameters are not given by way of limitation, but for purposes of illustration only. It should be noted that dimensions of the components comprising the barrel 11 may vary over a considerable range with respect to one another.

Opening 20, provided by flange 19, has a diameter of about 0.750 of an inch (1.90 cm). Opening 23 formed in tube 25 is spaced rearwardly about 0.350 of an inch (0.90 cm) from opening 20. Opening 23 has a diameter of about 0.460 of an inch 1.17 cm). Bullet-shaped cap 27 has a diameter of about 0.42 of an inch 1.07 cm) at edge 28 and a taper of about The axial length of cap 27 from rear surface 29 to the forwardmost tip is about 0.360 of an inch (0.90 cm). Tube 26 has an external diameter of about 0.400 of an inch (1.02 cm) and has its extremity 32 spaced about 0.015 of an inch (0.038 cm) from the rear surface 29 of cap 27. Thus it is seen that orifice 30 formed by the cooperative relationship between rear surface 29 and extremity 32 has axial width of about 0.015 of an inch (0.038 cm). The axial width of orifice 30 may be varied, as desired, by turning the threaded end 33 of cap 27 into or out of hub 35 cooperatively associated with tube 26.

A test fluid consisting essentially of about 1,500 milliliters of boiled linseed oil, about 900 milliliters of VM & P (Varnish Makers & Painters) naptha, about 900 milliliters of n-butyl alcohol, and about 350 milliliters of methyl alcohol provides approximately 1 gallon (3.8 liters) of sprayable liquid having a resistivity of about 6.5 megohm-centimeters. The test fluid is caused to flow from orifice 30 at flow rates of from about 150 to 540 milliliters per minute under gauge pressures of from about 15 to 20 pounds per square inch (2.0-2.4 atmospheres absolute).

The direct current input voltages supplied to electrode 31 are from about 4,000 to 7,000 volts and, preferably, about 4,000 volts. The input air pressure to the spray device 10 is about 35 pounds per square inch (3.4 atmospheres) at a flow rate of about 7 cubic feet per minute (200 liters per minute). The input current flowing from the ground terminal of the voltage source to ground of earth ranges from about 7.8 microamperes at a test fluid flow rate of about 150 milliliters per minute to about 14.0 microamperes at a test fluid flow rate of about 540 milliliters per minute.

The current which flows between the surface being coated with charged liquid particles and ground or earth ranges from about 8.5 microamperes at a test fluid flow rate of about 150 milliliters per minute to about 32 microamperes at a test fluid flow rate of about 540 milliliters per minute.

As a result of being formed in a region of concentrated high field strength, the liquid particles projected from the barrel of the spray gun carry a high charge-tomass ratio; the charge carried by the particles is opposite in polarity to the polarity of the electrode 31. It is seen that a cloud of charged liquid particles is provided in the space between the front of the spray gun l0 and the surface at a particle attracting potential. Such a cloud creates a space charge effect which assists in causing the deposition of those charged particles which are near the surface to be coated.

It should be understood that the size of individual liquid particles is determined by several factors including, the flow rate of liquid and air to the gun, the constituents of the liquid and the like. The average particle size should be in the range of about microns or less, and preferably, about 50 microns or less for quality paint finishes.

The barrel 11 illustrated in FIG. 2 may be modified so as to include suitable means for dividing or splitting the jet of air into two separate and distinct air flows; one of the air flows to sweep across the surface of electrode 31 to provide the cleaning action required and the other air flow capable of being adjusted so that it can provide more or less contribution to the atomization of particles from the termini of the stream of liquid at edge 28 of rear surface 29 of cap 27.

The charged spray particles projected into the space between the spray device 10 and the surface of an article at a spray attracting potential carry a polarity opposite the polarity of electrode 31 and exert a space charge effect thereby creating an electrostatic field extending to the surface of the article that assists in the electrostatic deposition of the charged particles in close proximity to the surface of such an article. If desired, the electrostatic depositing effect can be enhanced by providing the spray device with a spraysurrounding electrode 40 as shown in FIG. 3, which may be maintained at a deposition potential relative to the surface of the article at ground or earth potential. Such a spray surrounding electrode 40 functions so as to minimize the tendency of the spray of charged coating material particles to expand upon emerging from the opening of the spray device thereby facilitating control over the resultant spray pattern on the surface of the article. In addition, the spray surrounding electrode 40 functions to shield the electrode 31 from the high potential charged cloud of liquid particles having a polarity opposite from the polarity of the electrode 31. The spray repelling electrode is fabricated from any suitable metallic material such as steel or the like, or from dielectric material where the surface charge is induced thereon by the spray cloud.

Referring now to FIG. 4 of the drawing, another embodiment of the present invention is illustrated. Barrel 11 includes a substantially tubular sheath or housing 18. Housing 18 has a radial flange 19 at its fowardmost extremity. Flange 19 forms opening 20 through which a spray (not shown) of electrostatically charged liquid particles is projected from spray gun l0.

Substantially concentric outer tube 51 and inner tube 52 are located within housing 18 and are components of barrel 11. Preferably, tube 51 and 52 are coaxial with housing 18 and are fabricated from any suitable dielectric material capable of withstanding the stresses associated with high voltages without an accompanying breakdown or rupture of the material. Suitable dielectric materials include acetal resin, epoxy, glass filled epoxy and glass filled nylon. Tube 51 includes an integral, inwardly projecting flange 58 of determined extent that provides a substantially circular particle emitting orifice or opening 53.

Tube 52 includes at its extremity a reduced external diameter 60. The reduced external diameter 60 of tube 52 and the flange 58 of tube 51 cooperate to provide annular opening 54 from which a jet of air flows. Annular opening 54 communicates with the air source 15 through annular passageway 62 provided by the structural cooperation between tube 51 and 52. Tube 52 includes passageway 55 which terminates in orifice 56. A stream of liquid under low pressure issues from orifice 56 during operation of the spray gun 10. Preferably, the coating material flowing through passageway 55 of tube 52 is electrically grounded or earthed through head 63 in contact with the liquid in passageway 55 and spaced rearwardly a short distance from orifice 56.

An annular metallic electrode 57 of extended axial length is imbedded in tube 51 and is spaced rearwardly of opening 53 as shown in FIG. 4. Electrode 57 may be fabricated from any suitable conductive material such as alloys of copper. Preferably, electrode 57 is coupled through a current limiting resistor 61 of suitable ohmic value to power supply 22 in the spray gun 10 or on the person of the operator. It should be understood that electrode 57 is positioned within tube 51 and, hence, barrel 11 so as not to come in contact with any article to be coated and not to engage with an operator exercising usual operating caution, therefore, the current limiting resistor may be eliminated. However, good safety practice may dictate the use of a current limiting resistor of suitable ohmic value coupled in series between electrode 57 and the output terminal of power supply 22.

The voltage supplied to the annular electrode 57 by the power supply 22 through current limiting resistor 61 creates a high intensity electrostatic field extending across the annular air discharge opening 54 to the grounded or earthed terminus of liquid at orifice 56.

Supplying compressed air to passageway 62 at flow rates of up to 7 cubic feet per minute (200 liters per minute) and at gauge pressures of up to about 50 pounds per square inch-(4.4 atmospheres absolute) and causing liquid to flow from orifice 56 at a flow rate of up to about 280 milliliters per minute under a gauge pressure of up to about 5 pounds per square inch (1.3 atmospheres absolute) projects a spray of liquid particles having a high electrostatic charge from opening 23. The jet of air flowing in annular passageway 62 and between the annular electrode 57 and portion 60 of tube 52 causes the stream of liquid whose normal cross section is that of orifice 56 to expand and be formed into discrete termini upon which the field is concentrated. In addition, this air stream is used to direct the liquid particles away from the inner wall of the tube 51 and electrode 57. The jet of air contributes to the atomization of the liquid particles from the termini of the liquid issuing from orifice 56. It should be noted that the jet of air flows through an enlarged area 64 and then through a reduced area 65 prior to flowing from annular opening 54. This allows the jet of air to be accelerated in velocity just prior to expanding into the area adjacent the aperture 26 thereby insuring that a high velocity stream of air is between the outer edge of orifice 56 and electrode 57.

The diameter of orifice 56 of tube 52 and the distance between the electrode 57 and exterior surface 60 of tube 52 are each a small fraction of an inch (centimeter). For example, the separation between electrode 57 and the exterior surface 60 of tube 52 is on the order of about 0.035 of an inch (0.090 cm). The diameter of orifice 56 is on the order of 0.060 of an inch (0.150 cm). The axial extent of electrode 57 is on the order of0.250 ofan inch (0.640 cm).

Electrode 57 is connected to a direct current voltage source supplying about 10,000 volts 'or less, and, preferably, about 3,000 to 7,000 volts at its output terminal. Assuming a separation of 0.050 of an inch (0.177 cm) between the outer edge of the member surrounding orifice 56 and electrode 57 and a voltage of about 4,000 volts impressed on electrode 57 the average voltage gradient across the annular opening 54 is about 73,000 volts per inch (29,000 volts per centimeter). It is seen that a high intensity electrostatic field is provided to the liquid termini. As a result of being formed in a region of high field strength, the liquid particles emitted from the spray gun 10 bear or carry a high charge-to-mass ratio and carry a polarity opposite to the polarity of electrode 57.

The foregoing and the following dimensions and parameters are given to illustrate the operation of the barrel shown in FIG. 4 with the spray gun shown in FIG. 1. Such dimensions and parameters are not given in the way of limitation, but for purposes of illustration the forwardmost extent of opening 53. Orifice 56 has a diameter of about 0.060 of an inch (0.152 cm). Annular orifice 54 has a radial extent of about 0.035 of an inch (0.089 cm).

A test fluid consisting essentially of about 1,500 milliliters of boiled linseed oil, about 900 milliliters of VM & P (Varnish Makers and Painters) naptha, about 900 milliliters of n-butyl alcohol, about 300 milliliters of methyl alcohol and about 30 milliliters of diethylamine provides about 1 gallon of the fluid. The resistivity of the test fluid is about 1.6 megohm-centimeters'The test fluid is caused to flow from the orifice 56 at a flow rate of about 50 to 280 milliliters per minute at a gauge pressure of about 1 to pounds per square inch (1.07 to 1.34 atmospheres absolute).

The input air gauge pressure of the jet of air to the barrel 1] is about 30 pounds per square inch (3.0 atmospheres absolute) at a flow rate of about 3 cubic feet per minute (85 liters per minute). The direct current input voltage supplied to electrode 57 is about 4,000 volts. The input current appearing in the circuit from the ground terminal of the voltage source to ground or earth ranges from about 3.4 microamperes at a test fluid flow rate of about 50 milliliters per minute to about 8.2 microamperes at a test fluid flow rate of about 280 milliliters per minute. The liquid particle current or output current appearing between the surface on which liquid particles are being deposited and ground or earth ranges from about 2.8 microamperes at a test fluid flow rate of about 50 milliliters per minute to about 7.6 microamperes at a test fluid flow rate of 280 milliliters per minute.

Increasing the air pressure supplied to the barrel shown in FIG. 4 to about 50 psi gauge (4.4 atmospheres absolute) results in an input current flowing from the ground terminal of the voltage source to ground or earth in the range of about3 microamperes at a test fluid flow rate of about 50 milliliters per minute to about 3.6 microamperes at a test fluid flow rate of about 280 milliliters per minute. The liquid particles current or output current appearing between the surface on which liquid particles are being deposited and ground or earth ranges from about 2.5 microamperes at a test fluid flow rate of about 50 milliliters per minute to about 13.5 micromaperes at a test fluid flow rate of about 280 milliliters per minute.

While the invention is illustrated and described using several embodiments, it is to be understood that modifications and variations may be effected without departing from the scope and concepts of the invention.

Iclaim:

l. A low voltage electrostatic spray gun comprising a barrel including a passageway terminating in an opening,

the barrel including dielectric means for forming liquid into a stream, the stream having a plurality of termini in the passageway and spaced rearwardly from the opening, the liquid termini providing liquid particles,

electrode means confined within the passageway rearwardly of the opening and in close proximity with the liquid termini, the electrode means and the stream of liquid adapted to be connected to means for creating a potential difference between the electrode means and the liquid termini, the potential difference therebetween providing an electric field having a high potential gradient at the termini, the dielectric means assisting in concentrating the electric field at the liquid termini so as to electrostatically charge the liquid particles, and means in the barrel for directing a jet of air along the passageway to carry the charged liquid particles formed from the liquid termini away from the side walls of the passageway and the electrode means.

2. The low voltage electrostatic spray gun of claim 1, wherein the jet of air assists in atomizing particles from the liquid termini.

3. The low'voltage spray gun of claim 1, wherein the dielectric means forms the liquid into a stream having an axis that is substantially parallel to the axis of the electrode means.

4. The low voltage device of claim 1 wherein the dielectric means for forming the stream includes a tube and a cap spaced from the open end of the tube to provide an orifice from which the stream of liquid is emitted at an angle to the electrode, the cap having a radial extent slightly greater than the radial extent of the tube.

5. The low voltage device of claim 4, wherein the portion of the passageway in front and extending away from the cap has an expanding volume.

6. The low voltage spray gun of claim 1, wherein the electrode means is annular and the dielectric means forms the liquid into a stream having an axis substantially concentric with the axis of the annular electrode.

7. A low voltage electrostatic spray device comprismg a barrel having an opening,

a passageway in the barrel terminating in an opening near the opening of the barrel,

means in the barrel providing a liquid stream having termini from which liquid particles are formed in the passageway rearwardly of the opening of the passageway, the means providing the stream includes a cap spaced from an open end of a tube to form an orifice from which the stream of liquid is emitted, the cap having a radial extent different from the radial extent of the tube,

electrode means in the passageway and rearwardly of the opening of the passageway and in close proximity to the liquid termini,

means connected to the electrode means and the liquid stream to establish an electric field having a high gradient at the termini of the liquid, the electric field being concentrated at the liquid termini, and

means directing a jet of air along the passageway between the electrode means and the liquid termini for projecting electrostatically charged liquid particles generally away from the side walls of the passageway and out of the opening of the passageway and the opening of the barrel.

8. A low voltage electrostatic device as claimed in claim 7, wherein the means providing the liquid stream is fabricated from dielectric material to assist in concentrating the electric field at the liquid termini.

9. A low voltage electrostatic device as claimed in claim 8, wherein the voltage applied to the electrode means is of such a value to prevent corona discharge.

10. A method of forming a cloud of electrostatically charged liquid coating material particles comprising the steps of providing a liquid stream of coating material having a plurality of liquid termini in a passageway and spaced rearwardly from an end of the passageway,

creating a potential difference between an electrode means in the passageway and spaced rearwardly from the opening and the liquid termini, the potential difference therebetween providing an electric field having a high potential gradient at the liquid termini,

forming charged liquid particles from the liquid termini having a polarity different from the polarity of the electrode means, and

directing a jet of air along the passageway to carry the charged liquid coating material particles formed from the liquid termini away from the side walls of the passageway to form a cloud of electrostatically charged liquid particles.

11. The method of claim 10, wherein the liquid is paint having a resistivity of about 15 megohm-centimetcrs or less.

12. The method of claim 10, wherein the stream having a plurality of liquid termini is formed on a dielectric means.

13. The low voltage device of claim 1, wherein the dielectric means for forming the stream includes a tube and a cap spaced from the open end of the tube to provide an orifice from which the stream of liquid is emitted.

14. The low voltage device of claim 13, wherein the cap has a radial extent different from the radial extent of the tube. v

15. A low voltage electrostatic spray gun comprising a barrel having an opening,

a passageway in the barrel terminating in an opening near the opening of the barrel,

the barrel including dielectric means for forming liquid into a stream, the stream having a plurality of termini in the passageway and spaced rearwardly from the opening of the passageway, the liquid termini providing liquid particles,

electrode means confined within the passageway rearwardly of the opening of the passageway and in close proximity with the liquid termini, the electrode means and the stream of liquid adapted to be connected to means for creating a potential difference between the electrode means and the liquid termini, the potential difference therebetween providing an electric field having a high potential gradient at the termini, the dielectric means assisting in concentrating the electric field at the liquid termini so as to electrostatically charge the liquid particles, and

means in the barrel for directing a jet of air along the passageway to carry the charged liquid particles f ed from the li uid termini awa from the side vigil; of the passag ieway and electr de means and out of the opening of the passageway and the opening of the barrel.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated October 17, 1972 Patent No. 5, 9 55 Inventor(s) James E. Sickles It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 51, "System" should be Process Column 5, line 27, "barrel housing 18 and are components of 11" should be housing 18 and are components of barrel ll Column 8, line 2 5 "of" should be or led this 15th day of April 1.?7.

" i-med sea nest: c. tzxrszariii.

7W c 33c? Commission-qr of f'e'cents cw-r and Tradeiwr USCOMNPDC 60376-P69 U.S. GOVERNMENT PRINTING OFFICE:

F ORM PO-IOSO (10-69)

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Classifications
U.S. Classification239/3, 239/706
International ClassificationB05B5/025, B05B5/03, B05B5/043
Cooperative ClassificationB05B5/043, B05B5/03
European ClassificationB05B5/043, B05B5/03